Thermocouple element composition



Patented Got. 121, 1954 retreat THERMOCOUPLE ELEMENT COMPOSITION Stephen Pooh, Wyckoii', and John G. Lewis, Jr., Maplewood, N. 3., assignors to Driver-Harris Company, Harrison, N. 3., a corporation of New Jersey No Drawing. Application August 22, 1952, Serial No. 305,924

7 Claims. 1

This invention relates to thermocouples and, more particularly, to a combination of alloy compositions for the two components of a thermocouple which resist the deteriorative efiect of various furnace atmospheres at high temperatures.

The electromotive force developed by any alloy at any given temperature is sensitive to changes in the nature and quantities of the metallic constituents present in the alloy. There are other constituents too, of a non-metallic nature, which are invariably present in all industrial alloys and which, if varied, have a marked effect in changing the electromotive force of a given alloy combination. These non-metallic constituents include oxygen, nitrogen and hydrogen which are adsorbed by the metallic constituents of the alloy, and they further include compounds such as oxides, nitrides, hydrides and sulfides which are soluble to a significant extent in the matrix of which the alloy is composed.

It is quite possible, industrially, to manufacture alloys of definite composition which will produce a definite electromotive force within specified limits at a given temperature. Both metallic and non-metallic constituents can be readily controlled, the former by metal additions and the latter by oxidizing or deoxidizing agents used in the melting operation. As a result, the thermocouple alloys reach the ultimate consumer with properties conforming to the electromotive force which is desired. However, the consumer uses these thermocouples for temperature control under conditions which frequently tend to alter the composition of the thermocouple alloys. For example, the atmospheres in industrial furnaces are in most cases either oxidizing or reducing. When these atmospheres produce changes in the composition of the thermocouple alloy, as they invariably do at high temperatures, a corresponding change will occur in the thermal electromotive force of the alloy and the couple will depart from its initial calibration. Further changes due to the formation of oxides, carbides, and sulfides can occur. These metallic compounds, initially formed on the surface of the alloys, can dissolve in the alloys and diffuse in from the surface inasmuch as they tend to precipitate on grain boundaries within the alloy during the cooling cycle and serve as regions for further attack by the heated furnace atmospheres during a, subsequent heating cycle. Thus, in addition to changes in the electromotive force of the alloys, a general weakening of the structure results and the alloys develop brittle properties which destroy their usefulness.

Since both elements of any thermocouple contribute to the electromotive force of the combination, it is essential that both of them resist, to a high degree, the destructive changes which have been outlined hereinbeiore. Nothing is gained when one element of the thermocouple is highly resistant to attack from furnace atmospheres if the other is easily destroyed. The utility of the thermocouple is strictly limited to the service which can be obtained from the weaker member of the alloy combination.

A thermocouple which has become popular because of its electromotive force characteristics comprises a combination in which the electropositive element is a nickel-chromium alloy containing approximately 8 to 10% of chromium with other metallic additions in minor amounts and the electronegative element is a nickel alloy with a manganese content of approximately 3% and aluminum and silicon in amounts usually not exceeding 2% each. Both of these alloys have proven to be subject to attack at high temperatures by a variety of furnace gases to which they are exposed. In the original development of the nickel-chromium alloy ior thermocouple applications, a chromium content of approximately 9% was used because this addition gave the maximum positive electromotive force in the nickel-chromium alloy system, and the stability of the alloy when exposed at high temperatures to furnace gases received subordinate consideration. We have now found that nickel-chromium alloys having higher chromium contents, when used as the electropositive element of a thermocouple in combination with a certain other alloy composition as the electronegative element, result in thermocouples characterized by stability of their high electromotive force in the presence of corrosive atmospheres at elevated temperatures.

The novel thermocouple of our present invention comprises an electropositive element composed of an alloy containing from 10 to 25%, and preferably 18 to 22%, chromium and the bal ance substantially all nickel and an electronegative element composed of an alloy containing from 2 to 7%, and preferably 2 to i%, silicon and the balance essentially all nickel. Presently preferred compositions within the aforementioned ranges comprise electropositive elements containing from 18 to 22% chromium, up to 2% columbium, up to 2% iron, up to 2% manganese, from 0.5 to 2% silicon, from 0.01 to 0.15% carbon, not more than 1 other metals than nickel, and the balance nickel, and electronegative elements containing 2 to 4% silicon, 0.01 to 0.15% carbon, and the balance substantially all nickel except for incidental impurities normally associ ated with nickel. 7

Within the broad range of useful alloy compositions which may constitute the electropositive element of our novel thermocouples, we have found that, regardless of the presence or absence of other incidental constituents, increasing amounts of chromium within the range of to 25% increase the resistance of the alloy to deterioration at high temperatures in industrial furnace atmospheres but decrease the electromotive force of the alloy. Thus, the alloy containing 10% chromium has a higher electromotive force but lower deterioration resistance than an alloy containing 25% chromium. We have ascertained that a chromium content of about results in an optimum combination of these two characteristics, particularly when used in conjunction with the electronegative siliconcontaining nickel alloy component of the thermocouple.

Although the chromium-containing alloys of the electropositive element of our novel thermocouple are stable and resist attack by strongly oxidizing and strongly reducing furnace atmospheres, even the higher-chromium alloys are somewhat subject to what is known in the art as green rot deterioration when the furnace atmosphere is of a character intermediate these extremes or when the atmosphere is alternately oxidizing and reducing in character. Such green rot can be minified or even eliminated by the addition of 0.2 to 2% of columbium or silicon, or both, to the alloy composition of the electropositive element of our novel thermocouple. Small amounts of other metals such as iron and manganese in amounts up to 2% and carbon in amounts from 0.1 to 0.15% may also be present. The electronegative thermocouple elements which in the past have generally been used with a chromium-containing nickel electropositive element have been even more susceptible to deterioration than the electropositive element and have been particularly susceptible to deterioration in some reducing atmospheres. In such atmospheres, at high temperatures, these prior art electronegative elements absorb reducing gases which, on alternately heating and cooling, cause precipitation of extraneous compounds within the grain boundaries of the alloy with resulting grain boundary embrittlement. Our investigations have disclosed that additions of 2 to 7% silicon to nickel minify or eliminate the embrittlement which otherwise results from exposure to reducing furnace atmospheres, that in oxidizing atmospheres this nickel-silicon alloy gives wholly satisfactory service, and that in combination with an electropositive element containing from 10 to chromium and the balance substantially all nickel the resulting thermocouple has an effective electromotive force which is substantially unaltered by extensive exposure of the thermocouple to both oxidizing and reducing atmospheres at high temperature. Since processing problems increase as the silicon content of the alloy is raised, we presently prefer to restrict the silicon content of the electronegative element to between 2 and 4%. Small amounts of carbon, say 0.01 to 0.15%, may be advantageously incorporated in the silicon-containing element, although we have not ascertained that the addition of any other extraneous metals has any beneficial effect. Thus, except for incidental amounts of other metals normally present as impurities in nickel, we have found that the alloy preferably should be composed substantially exclusively of nickel and silicon plus the aforementioned amount of carbon.

The relation between temperature and electromotive force in a thermocouple of our invention characterized by outstanding resistance to deterioration at high temperature in conventional furnace gas atmospheres is illustrated in Table I, the electropositive element consisting nominally of 19.5% chromium, 0.05% carbon, 0.8% silicon, 1.1% columbium, 0.05% manganese and the balance substantially all nickel and the electronegative element consisting nominally of 3% silicon, 0.02% carbon and the balance substantially all nickel.

Table I Electromotive Temperature (F) Force (Millivolts) The stability of the electromotive force in 'a thermocouple of our invention at an elevated temperature in different furnace atmospheres is illustrated in Table II. In this table, thermocouple A embodied our present invention and was composed of an electropositive element consisting nominally of 19.5% chromium, 0.05% carbon, 1.2% silicon, 0.5% manganese .and the balance substantially all nickel and an electronegative element consisting nominally of 3 silicon, 0.02 carbon and the balance substantially all nickel, and thermocouple B was representative of prior art alloy compositions and was composed of an electropositive element consisting nominally of 10% chromium and nickel and an electronegative element consisting nominally of 5% total of aluminum, manganese and silicon and nickel. Both thermocouples were tested in the indicated atmospheres after exposure for '18 hours at 2100 F. in the atmospheres indicated.

Table II EXPOSURE 'IN AIR AND TESTED IN AIR M2. M21. After 24. 88 After 34. 87 Before 24. 79 Before 34. 42

Change +0. 09 Change. +0. 45

EXPOSURE IN BURNED FURNACE GAS AND TESTED IN CITY GAS M1). Mu. After 24. 86 After 34. 86 Before M. 79 Before 34. 42

Change +0. 44

Change +0.07

EXPOSURE IN BURNED FURNACE GAS AND TESTED IN AIR Mv. M1 After 24. 82 After 34. 89 Before 24. 79 Bcfore34. 42

Change +0.03 Change +0. 47

It will be appreciated, accordingly, that the novel thermocouple of our invention is not only resistant to attack by corrosive atmospheres, and hence stable under normal operating conditions, but is characterized by an electromotive force that adapts the thermocouple admirably to accurate temperature measurement of commercially encountered furnace atmospheres.

We claim:

1. A thermocouple comprising an electropositive element composed of an alloy containing from to 25% chromium and the balance substantially all nickel and an electronegative element composed of an alloy containing 2 to 7% silicon and the balance essentially all nickel.

2. A thermocouple comprising an electropositive element composed of an alloy containing 18 to 22% chromium and the balance substantially all nickel and an electronegative element composed of an alloy containing 2 to 4% silicon and the balance essentially all nickel.

3. A thermocouple comprising an electropositive element composed of an alloy containing from 10 to 25% chromium, up to 2% columbium, and the balance substantially all nickel and an electronegative element composed of an .alloy containing 2 to 7% silicon and the balance substantially all nickel.

4. A thermocouple comprising an electropositive element composed of an alloy containing from 18 to 22% chromium, up to 2% columbium, and the balance substantially all nickel and an electronegative element composed of an alloy containing 2 to 4% silicon and the balance substantially all nickel.

5. A thermocouple comprising an electropositive element composed of an alloy containing 18 to 22% chromium, up to 2% columbium, up to 2% iron, up to 2% manganese, 0.5 to 2% silicon, 0.01 to 0.15% carbon, not more than 1% of other metals, and the balance substantially all nickel and an electronegative element composed of an alloy containing 2 to 4% silicon, 0.01 to 0.15% carbon, and the balance substantially all nickel.

6. A thermocouple comprising an electropositive element composed of an alloy consisting of 19.5% chromium, 0.05% carbon, 1.2% silicon, 0.5% manganese, and the balance substantially all nickel and an electronegative element composed of an alloy consisting of 3% silicon, 0.02% carbon, and the balance substantially all nickel.

7. A thermocouple comprising an electropositive element composed of an alloy consisting of 19.5% chromium, 0.05% carbon, 0.8% silicon, 1.1% columbium, 0.05% manganese, and the balance substantially all nickel and an electronegative element composed of an alloy consisting of 3% silicon, 0.02% carbon, and the balance substantially all nickel.

References Cited in the file of this patent UNITED STATES PA'I'ENTS Number Name Date 1,076,488 Marsh Oct. 21, 1913 2,306,290 Widell Dec. 22, 1942 2,587,391 Seaver Feb. 26, 1952 FOREIGN PATENTS Number Country Date 438,019 Germany Dec. 3. 1926 OTHER REFERENCES American Institute of Physics, 1941, page 1227. Materials and Methods, No. 169, November 1948, page 97. 

1. A THERMOCOUPLE COMPRISING AN ELECTROPOSITIVE ELEMENT COMPOSED OF AN ALLOY CONTAINING FROM 10 TO 25% CHROMIUM AND THE BALANCE SUBSTANTIALLY ALL NICKEL AND AN ELECTRONEGATIVE ELEMENT COMPOSED OF AN ALLOY CONTAINING 2 TO 7% SILICON AND THE BALANCE ESSENTIALLY ALL NICKEL. 